1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor thin film, and an image display device. More particularly, it relates to a manufacturing method for acquiring a high-quality semiconductor thin film by irradiating with laser light and crystallizing a semiconductor thin film formed on an insulating substrate. Accordingly, the present invention is a one suitable for active-matrix-scheme image display devices.
2. Description of the Related Art
The active-matrix-scheme image display devices (which, otherwise, are referred to as “active-matrix-type driving scheme image display devices”, or as merely “display devices”) are widely used at present. In the active-matrix-scheme image display devices, active elements such as thin film transistors (TFTS) are used as driving elements for driving matrix-arranged pixels. In many of this type of image display devices, a large number of pixel circuits and driving circuits are configured with the active elements such as the thin film transistors formed using a silicon film as the semiconductor film. Moreover, these large number of circuits are located on an insulating substrate, thereby making it possible to display excellent-quality images. Here, the explanation will be given selecting, as an example of the active elements, the thin film transistors, i.e., the typical example thereof.
In the thin film transistors using a non-crystalline semiconductor film (amorphous semiconductor film) which has been generally used as the semiconductor film so far, there exists a limit to performances of the thin film transistors the representative of which is their carrier (electron or hole) mobility. This limit has made it difficult to configure circuits requested to exhibit high-speed and high-function performances. In order to implement thin film transistors having high mobility needed for providing superior image quality, it is effective to modify property of (i.e., crystallize) the amorphous film (hereinafter, also referred to as “non-crystalline film”) to a polycrystalline film (hereinafter, also referred to as “polycrystal film”) in advance, and to form the thin film transistors by using the polycrystalline film. For implementing this property modification, a method is used which anneals the amorphous film by irradiating the amorphous film with laser light such as excimer laser light.
Referring to FIG. 24A and FIG. 24B, the explanation will be given below concerning the property modification method based on the crystallization of an amorphous-silicon film by using the excimer laser-light irradiation. FIG. 24A and FIG. 24B are explanatory diagrams for explaining the crystallization method for crystallizing an amorphous-silicon film by scanning the most common excimer pulse laser-light irradiation. FIG. 24A illustrates configuration of an insulating substrate on which the semiconductor layer to be irradiated is formed. FIG. 24B illustrates a state whose property is to be modified by the laser light irradiation. As this insulating substrate, a glass or plastic substrate is used.
In FIG. 24A and FIG. 24B, an amorphous-silicon film ASI, which has been deposited on an insulating substrate SUB via a buffer layer (SiN, SiO2, or the like, not illustrated), is irradiated with about several-nm to several hundred-nm-wide line-shaped excimer laser light ELA. Then, a scanning is performed that the irradiation position is displaced on each one to several-pulse basis along one direction (X direction) as is indicated by the arrow. This scanning anneals the amorphous-silicon film ASI, thereby modifying property of the amorphous-silicon film ASI on the entire insulating substrate SUB to a polysilicon film PSI. Moreover, various machinings, such as etching, wiring formation, and ion implantation, are performed to the polysilicon film PSI acquired by the property modification in this method. This allows formation of the circuits which include active elements such as thin film transistors in each of the pixel units or driving units.
Taking advantage of this insulating substrate, the active-matrix-scheme image display devices, such as liquid-crystal display devices or organic EL display devices, are manufactured. In the conventional property modification of the amorphous-silicon film using the excimer laser light, a large number of about 0.05-μm to 0.5-μm-diameter crystallized silicon grains (i.e., polysilicon) grow at random in the laser-light-irradiated portion. The electric-field effect mobility of the TFTs formed of the polysilicon film like this is equal to about 200 cm2/V·s or less, and about 120 cm2/V·s on average.
Furthermore, as a method for acquiring the high-quality semiconductor thin film, there exists the following technology: Namely, as is disclosed in JP-A-2003-86505, a semiconductor thin film is irradiated with continuous-wave laser (CW laser) while scanning the continuous-wave laser in one direction. This irradiation grows a crystal which is continuous in the scanning direction, thereby forming the crystal which extends long in that direction. Moreover, the substrate is scanned while irradiating with the CW laser a semiconductor thin film which has been machined in advance in an island-shaped or line-shaped manner. Otherwise, a thermal gradient is given at the time of the laser annealing. These methods make it possible to acquire a crystal (hereinafter, referred to as “lateral crystal”) which is flat and whose crystalline grains have grown significantly in one direction. As an example using the continuous-wave laser other than this document, there exists JP-A-2003-124136. Meanwhile, in addition to JP-A-2003-86505 and JP-A-2003-124136, there also exists a method for inducing the lateral crystal by giving a thermal gradient through irradiation of the film with the ELA via a slit or mask whose slit width is smaller than several μm. Applying the semiconductor thin film like this allows acquisition of a high-performance characteristic that the electric-field effect mobility is larger than about 300 cm2/V·s.